An image processing apparatus that generate color printing data CMYK (K: black) from color data CMY (cyan, magenta, yellow) is disclosed, for example, in Japanese Patent Laid-open Application No. 11-164164. This image processing apparatus detects the density of the black color from the CMY binary color data, and generates image patterns for black generation processing (BG processing) and under-color removal processing (UCR processing) based on the detected density, followed by generating the CMYK color print data based on the generated patterns. Some related technologies are disclosed in Japanese Patent Laid-open Applications No. 5-276368 and No. 10-200765, for example.
A straightforward method of generating K from the CMY binary data would be to calculate K=C&M&Y. In this method, however, K is generated at all pixels where CMY is 1 indicative of a gray color. If all the C, M, Y, and K colors are printed in areas where the density of the gray color is high (i.e., areas where the density of K pixels is high), an excessive amount of toner would end up being applied. There is thus a need to perform the UCR processing to remove CMY in areas where K pixels are generated. If the UCR processing is performed after generating the K pixels, however, all the gray pixels where CMY is 1 are changed to pixels having only the K color. As a result, a reproduced image ends up lacking rich color.
Namely, all the gray pixels are subjected to the BG processing that generates K, and end up lacking rich color because of the UCR processing performed thereon.